1. Field of the Invention
This invention relates generally to mechanical pipe joints, and, more particularly to mechanical pipe joints formed between sections of metal pipe or tubing by forcing a pin section on the end of one pipe section into a socket formed on the end of another pipe section and to methods for sealing these joints to improve the performance and reliability of mechanical pipe joints under the most severe field conditions.
2. Background Art
In many applications, mechanical pipe joints have advantages over other conventional types of pipe joints. They require no welder in the field and therefore a pipeline can be laid using this type of joint with relatively inexpensive, unskilled workers. Also, no alignment clamps are required, as in the case of welded joints. Using this type of pipe joint, instead of a threaded pipe joint, eliminates the problem of thread protection while moving the pipe to the field and stringing it along the right of way. In most circumstances and conditions, a mechanical pipe joint can be completed many times faster than a threaded or welded joint. Such a joint is particularly advantageous for use with internally coated pipe because forming the joint will not damage the coating and any gap between the coatings on the two joined pipe ends is filled with a sealant which is used to seal the joint.
Mechanical joints are formed by forcing a pin section on the end of one pipe section into a socket formed on the end of another pipe section. The socket includes a cylindrical section, which has an inside diameter slightly less than the diameter of the pin, to form an interference fit between the pin and cylindrical section of the socket when the joint is assembled. The interference fit provides structural strength to the joint.
The socket also includes a conical or tapered section between the cylindrical section and the pin section. The socket is formed by inserting a mandrel of a predetermined size into the pipe to bell the end of the pipe outwardly to form both the cylindrical section and the tapered section. The pin section of the joint requires only the forming of a tapered section on the end of the pipe.
Butler, U.S. Pat. No. 4,095,825, commonly owned with the present application, and which is hereby incorporated by reference herein in its entirety, discloses a tapered pipe joint wherein the tapered section of the socket and the tapered section of the pin are provided with diverging taper angles which result in an open-ended annular cavity being formed between the tapered sections when the pin was fully inserted into the socket. Sealant is trapped in the annular cavity between the diverging tapers and the socket and pin to form a seal in the cavity. Although this mechanical pipe joint is effective, the open-ended annular cavity formed by the diverging tapered angles creates a weakness in the seal of the joint that increases the probability of leakage over time and is ineffective in providing a “holiday” free pipe surface for internally coated pipe. A “holiday” is a defect, discontinuity, or thin spot in a coating.
Butler, U.S. Pat. No. 4,696,499, commonly owned with the present application, and which is hereby incorporated by reference herein in its entirety, discloses a mechanical pipe joint and method of forming the same, which was an improved mechanical pipe joint wherein the tapered sections on the socket and the pin, respectively, have converging taper angles that form a substantially closed annular cavity in which sealant is trapped to provide a seal for the joint. Additionally, this patent provides a seal and a coating technique that protects mechanical pipe joints in an internally coated pipeline from erosion by the corrosive fluids flowing through the pipeline. This patent improves the sealing area by the application of converging taper angles that create a substantially closed annular cavity in which sealant is trapped to provide a seal for the joint. It also provides a workable method for utilizing mechanical pipe joints with internally coated pipe. The mechanical joint configuration of this patent, however, has several disadvantages that become apparent in the most demanding and severe field applications. First, the seal is created by the contact of two pipe sections with converging angles. Accordingly, the thickness of the seal is limited to the thickness of the portion of the end of the taper section of the pin which actually comes in contact with the taper section of the socket. This thickness is many times less than the width of the pin and may result in a penetration into the wall of the socket. Second, the surface configurations of the sections of the pin and socket that create the annular cavity fail to take advantage of the exothermic properties of the latest acrylated synthetic polymers used as sealants under severe cold weather conditions. Third, the bottom section of the tapered end of the pin extends downwardly past the inside surface of the pipe creating a small obstruction that can create turbulence in the area of the mechanical pipe joint as fluids move through the pipeline.
Various other systems and methods have been devised to join the ends of pipes, most of which have the problems and disadvantages mentioned above. For example, Meagher, U.S. Pat. No. 3,909,045, discloses a plastic pipe joint configuration for use with adhesives that includes a female member having a retaining area and a male member having a piston means for applying pressure to the adhesive during telescoping of the members to hydraulically force the adhesive into the retaining area to eliminate voids and provide a leak-tight joint. The female member is provided with axially spaced flanges at each end of the retaining area that are adapted to be engaged when the members are fully telescoped by a mating surface on the free end of the male member and by the piston means to seal the void-free adhesive in the retaining area. Joining plastic pipe using adhesives, however, is generally unacceptable due to the difficult adherence characteristics of many polymers, which can result (after a period of time) in cracks in the cured adhesive or in separation of the adhesive from the pipe surface. The use of adhesive or glue also often involves a change in the inner diameter or outer diameter of one or both of the plastic pipes, such as with a bell-and-pipe slip joint. Non-uniform pipe diameters can introduce difficulties when installing the pipe.